When your classic Mopar came from the factory, the live axle, or solid rear axle rear, was the OE choice. A live axle consists of a centrally-located differential within a single housing. This housing also contains the axles that connect the differential to the wheels. The differential is connected to the transmission by a driveshaft that has a universal at each end. The reason for the universals is because when the rear suspension moves, this allows the rear end to also move, and the universals must be in place to compensate for the angle changes that the driveshaft sees during these movements. The main advantage of using a solid axle is that it is simple and relatively inexpensive to manufacture. Its drawbacks, however, are that it doesn't allow each wheel to move independently in response to bumps, and the weight of the assembly is part of the vehicle, which can limit ride quality.
Because a solid axle doesn't exhibit camber change as the suspension travels, they are ideal for carrying heavy or varying loads. Although this negatively impacts cornering as compared to other suspension designs (because the wheels have zero camber gain during body roll).
Independent rear suspensions typically offer a better ride quality and handling characteristics because of their lower unsprung weight, and the ability of each wheel to remain in contact with the road when the opposite side experiences an uneven surface (read: pot hole). Independent rear suspensions do, however, require a significant amount of additional engineering and expense in development.
The reason for the lower unsprung weight of an IRS, as opposed to a solid axle, is that the axle does not mount to the springs or suspension system. Instead, it is either bolted directly to the vehicle's chassis, or--more commonly--to a subframe.
The relative movement between the wheels and the differential is achieved through the use of swinging driveshafts connected by constant-velocity (CV) joints.
Since the movement of each rear wheel is not affected by the other, the vehicle is able to maintain traction where a solid-axle equipped vehicle might lose contact with the driving surface. This also works to maximize tire performance, as independent suspensions allow the tread to contact the road over a great surface area, allowing the tire to wear more evenly and extend life.
Independent rear suspensions do offer both pros and cons when it comes to performance usage. While it's true that an independent rear suspension gives a car better cornering ability, since there is less unsprung weight to pull against the driver's steering, IRS cars will usually see a lower performance outcome when racing in a straight line.
Independent rear suspensions became popular among custom hot rod designers just as they were appearing on factory sports car models. For these do-it-yourself car designers, an independent rear suspension was also a chance to give a vehicle a unique look. Even today, hot rods often include elaborate, chrome-plated rear suspensions that are easily visible to drivers behind the car, making a visual statement and showing the designer's attention to detail.
01 Since this is a basic...
01 Since this is a basic universal kit, the brackets will need some fabrication work. The piece shown here is the passenger-side bracket that holds the main differential mount to the car.
02 The actual cross-mount...
02 The actual cross-mount and the differential are part of the kit. With an IRS, the differential is rigidly mounted to the body.
03 Mounting the upper links...
03 Mounting the upper links to the differential is done by this supplied bracket. With a triangulated four-link, the upper two links (control arms) are closer together at the differential, and farther apart at the front crossmember. The upper links keep the axle from rotating, and keep the pinion angle constant. The "triangulated" upper bars also keep the rear end centered under the car so a Panhard rod is not needed. One downside to a triangulated system is called roll bind. This is where there is a lateral-corning force that is applied to the rear end, and basically binds the action of the suspension movement. This can, however, be controlled by using strong, high-quality heim joints or urethane bushings--which is what Heidts uses--that have much less deflection than rubber bushings.
04 The forward crossmember...
04 The forward crossmember for mounting the upper control arms is also a fabricated piece by KC's. Since this is a universal kit, you will need to do some fabricating. Hopefully the guys at KC's and Heidts can get together and make this a true Mopar-specific kit.
05 The lower control arms...
05 The lower control arms are next. The arms are fully-welded tubular units with polyurethane bushings.
06 When mocking the kit into...
06 When mocking the kit into the car, you need to make sure that you set everything at ride height. We used these pieces of square tubing to simulate the shock and coilovers to give us our height.